that looks great... just compensate for the usless hump in the rear of the cab and make a mach-up makesure it fits comfortably... how much volume do you need in the box?
I'd go with a 10. I have a 10 with a 1cuft ported box making 500W RMS and it sounds good especialy if you extend the box to add a 6X9 on either side of the sub (in independant sections of the box of course)